One of the early events in the response of cells to growth factors, hormones, immune cytokines, and neuropeptides is the stimulation of protein serine/threonine phosphorylation, due to the activation of several protein serine/threonine kinases. In recent years, attention has been focused on one particular group of these kinases that appears to represent a common intermediate in many signal transduction pathways because the kinases have been found to be stimulated by a wide variety of extracellular signals (i.e., including insulin, EGF, PDGF, NGF, serum, PMA, nicotine, okadaic acid, dibutyryl cAMP, and bradykinin), in numerous types of cells including epithelial, neural, and immune cells). The latter group of protein kinases is variably referred to as microtubule-associated protein-2 kinase (1, 2), myelin basic protein (MBP) kinase (3), mitogen-activated protein (MAP) kinase (4), and extracellular signal-regulated protein kinases (ERKs (5)), as well as by other names (for a review see citations 6-8). One form of the enzyme (ERK1) has reportedly been purified (9), and its cDNA sequence has been reported (5). Other isozymes may also be present in mammalian tissues, as well as in other organisms (7, 10). Accumulating evidence suggests that MAP kinases play an important role in cellular signaling by phosphorylating a wide variety of important substrates, including regulatory proteins such as c-jun (11), c-myc (12), Raf-1 (13), and the EGF receptor (14); dendritic axonal process substrates involved in microtubule rearrangement for neurite outgrowth (67); pp.sup.90.spsp.rsk ribosomal S6 kinase (15-17); and bradykinin (21) a G-protein-linked signal transduction.
Several models have been proposed for the mechanism of action of the MAP kinase activator(s) (19-22). These include the possibility that the MAP kinase activator(s) is a single protein kinase that catalyzes the phosphorylation of both tyrosine and threonine residues or that it is a kinase that catalyzes the phosphorylation of only one of these residues, and this is then followed by autophosphorylation of the other. Another possibility is that the MAP kinase activator(s) is not a kinase but is a protein factor that increases the rate of autophosphorylation of MAP kinase on tyrosine and threonine residues. The last possibility was supported by the observation that ERK1 and ERK2 are capable of autophosphorylation on tyrosine and threonine residues (22-25), a reaction that is accompanied by autoactivation (22,25), and by the fact that no substrates for the MAP kinase activator(s) other than MAP kinase has been detected (19).
Two forms of MAP kinase activating proteins have been reported in Swiss 3T3 cells. These activating proteins, which were present in their active forms only after EGF stimulation of the cells, promote phosphorylation of MAP kinases on threonine and tyrosine residues (19). In addition to their activation by EGF, the MAP kinase activators appear to be stimulated by phorbol esters (19) and possibly related components have been identified in PC12 cells (20,21), where they are activated by nerve growth factor or bradykinin (21). The ability of the MAP kinase activators to be activated by the various factors suggests that they may play an important role in the growth factor-stimulated kinase cascade (19).